In the measurement of ballistic projectile velocity, it is common to use a pair of photodetectors spaced a known distance apart to signal to a timer that the projectile has passed each detector. The interval of time measured by the timer and the known distance are then used to compute an instrumental velocity. The photodetectors normally operate by having the receiving elements (typically photodiodes or phototransistors) placed so that they receive steady illumination from incandescent lamps or another steady light source such as the sky or other broad source of light. The receiving element, the light source and the expected projectile path are arranged so that the projectile passes between the light source and the receiving element. The passage of the projectile causes a momentary decrease in the light intensity received by each receiving element. The electrical response of each receiving element is amplified and processed to provide appropriate start and stop signals to the timer.
The photodetector assemblies are typically called "screens" in the ballistics field because they replace physical conducting grids or screens which were used in earlier devices to signal passage of a projectile. Screens utilizing incandescent light sources were typically called "Lumiline" screens because the lamp most commonly used was a Lumiline lamp manufactured by the General Electric Company and others. Screens utilizing ambient light from the sky viewed through an optical slit or other mask were commonly called "sky screens". At the present time, the screens represent the most critical part of a velocity measuring system. The present state of the art in counting and computation circuits required for the timing and time-to-velocity conversion are far ahead of the technology of the screens.
A typical problem encountered in the fabrication and operation of photodetector assemblies for ballistics use is that the illumination received by the receiving element is not constant because of varying voltage to the incandescent lamp or lamps, varying amounts of dust in the light transmission path between the light source and the receiving element, aging of the incandescent lamp, varying distances from the light source to the receiving element, and varying cloud and atmospheric conditions and sun angles in the case of sky screens. Additionally, there are problems with the receiving elements because the amount of signal current produced by a receiving element under quiescent light conditions is a function of very broad manufacturing tolerances on the receiving element itself, the alignment of the optical system, and any aging characteristics of the receiving element. Still further, extraneous light sources may reflect from the projectile and cause abnormal behavior. For example, a strong light from the side may reflect from the projectile and cause the receiving element to preceive an increase in light level instead of the anticipated decrease.
Various efforts have been made to overcome these problems. For example, regulated voltage is provided to lamps to provide a steady light output in the presence of fluctuating power line voltage. Automatic gain control amplifiers are used to sense the amount of illumination provided by the lamps and to adjust the gain of amplifiers to compensate for changes in illumination level. Another approach is to select receiving elements for appropriate sensitivity, or to provide manual gain controls which the user can adjust to establish initial conditions. These various efforts are only partial solutions and are relatively complex and expensive, particularly when used in conjunction with each other.